Details:
Buy on Steam:
- get no hats for TF2 (yet)
- get 10% off right now
- get instant beta access in a week (August 14th) to play for a week until its release (August 21st)
- play on OS X and Windows
- play with much more people online
- mods and custom crap
Buy on PSN:
- PlayStation®Plus discount
- buy the other unreleased games in the collection/bundle it's involved in and get a discount which stacks with PS®Plus discount
- might have Steam integration like Portal 2 when you press SELECT?
- nothing for TF2 for PS3 because TF2 for PS3 has nothing
Buy on XBLA: